Epidemiologic studies of the impact of traffic-related air pollution (TRAP) on human health are frequently limited by the use of surrogate, ambient, or modeled exposure. The need for personal monitoring is evident in susceptible subpopulations, particularly children, who spend significant time at schools and in vehicles, in addition to their homes. Further, children may have high levels of physical activity and be in close proximity to particulate sources that result in higher levels of exposure than those recorded at stationary sampling sites. In order to address current limitations in characterizing exposure, a personal and wearable sensor capable of measuring, with high spatiotemporal resolution, exposure to ultrafine particles (UFP) has been developed and validated in laboratory settings. Though the personal sensor was developed to be sufficiently small, rugged, and straightforward to allow its widespread use in epidemiologic studies, it has not been validated outside the laboratory or utilized in the field. Therefore, the objectives of this application are to validate this highly innovative sensor in the field and conduct a field test of the sensor in an ongoing epidemiologic study of TRAP exposure at schools and communities. This study will benefit from the collaboration of engineers and epidemiologists to produce a personal sensor for UFP capable of being utilized in epidemiologic studies. In order to achieve the study objectives, the developed sensor will be validated in the field against current, non- personal, gold-standard instruments. Following the validation of the sensor, a field test of the personal sensor will be conducted within the context of a currently ongoing epidemiologic study, the Cincinnati Anti-Idling Campaign (CAIC) study. Children enrolled in the CAIC attend one of four participating Cincinnati Public Schools with varying exposure to TRAP from nearby roads or school buses. In addition, CAIC participants are asthmatic and have completed an initial health assessment. Exposure to TRAP is currently assessed in the CAIC study through the use of school and community ambient air monitoring. Thus, the CAIC study is ideal to assess the benefit of personal monitoring and to determine the acceptability and usability of the developed sensor in a population of inner-city asthmatic children. The results of the field test will provide feedback in order to modify the sensor in preparation for wide-spread use in epidemiologic studies.